| Vehicle seat active suspensions have been a hot research topic for many yearsdue to their impact on the drivers'fatigue, health and discomfort. In this paper, threehot issues of vehicle seat active suspension system are considered, i.e., input delayedcontrol, actuator saturation control and fault tolerant control, and corresponding con-troller design methods are given, respectively.Using electrohydraulic actuators to track the desired forces is inevitably involvedwith input time delays, which often degrade the control performances and even causeinstability in control systems. This paper deals with the problem of input-delayedrobust H_∞state-feedback controller design for a class of semi-active seat suspensionsystems with parameter uncertainties and actuator time delays. The H_∞norm fromthe disturbance to the controlled output is minimized under necessary time-domainconstraints. Besides, the delay partitioning method is employed to reduce the conser-vatism.Moreover, actuator saturation appears frequently in engineering systems, whichis also a source of performance degradation and instability. By employing a delay-range-dependent Lyapunov function and exploring the property of the saturation non-linearity, the existence conditions of the desired state-feedback controller are derivedin terms of linear matrix inequalities. The controller is then derived and the closed-loop system is asymptotically stable with a guaranteed H_∞performance.This paper is also concerned with the problem of robust H_∞reliable load-dependent controller design. A four degree-of-freedom human body model is consid-ered in order to investigate the control strategy more precisely. The load-dependentapproach is based on a parameter-dependent Lyapunov function. Thus the proposedmethod is capable to reduce the conservatism and improve the closed-loop perfor-mance. The control systems are reliable because they provide guaranteed asymptoticstability and H_∞performance in spite of the possible actuator failures.
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